Today liquid fuel components are mainly based on crude oil. There is an ever growing demand for liquid fuels with lower CO2 emissions compared to crude oil based fuels. Various renewable sources have been used as alternatives for crude oil fuels.
Vegetable oils and animal based fats can be processed for use as liquid biofuels in the form of fatty acid esters or hydrocarbons. Lipids for use in biofuels can also be produced in microorganisms such as algae, fungi and bacteria.
A typical problem with the use of animal based fats or vegetable oils, in particular microbial oils for liquid fuel production, is that they tend to contain significant amounts of metal and phosphorus impurities. These undesirable impurities are difficult to remove from renewable source material without simultaneously removing some of the valuable components. The impurities cause problems, for example, in the fuel production in form of catalyst poisons and/or corrosive materials. Deposits of metal and phosphorus compounds are likely to result in catalyst deactivation and plugging of the reactor catalyst bed in refining processes. In addition to phosphorus and metals animal fats frequently further contain thousands of ppms nitrogen which is hard to remove by existing pretreatment procedures.
Therefore, it is often required to use pretreatment steps or precleaning for removal of these undesired components from the oil product. Common treatment methods such as water degumming, soft degumming, acid degumming, wet bleaching and dry bleaching, for example, are able to remove most of the phospholipids and their salts from the feed stream. A disadvantage in using these methods is that a notable amount of feed which could be reformed into fuel is lost. In a degumming process especially phospholipids as well as metal impurities are removed in the form of gums. The formed gums contain significant amount of lipid material in the form of complex lipids thus decreasing the yield in fuel production. Other compounds used in oil purification like bleaching earth may become annoying waste that is difficult and expensive to handle, and simultaneously valuable agricultural fertilizer components are lost.
Microorganisms such as algae, archaea, bacteria and fungi including filamentous fungi and yeast may contain triglycerides up to 80% of their total dry matter content. However, oil from microbial biomass which is suitable as precursor for fuel production is scarce on the market. This is mainly due to lack of efficient and economical methods for providing good quality oil from microbial biomass. The typical drawbacks are high impurity contents and/or low yield.
When microbial biomass is used as feedstock the high amount of phospholipids i.e. membrane lipids from the total lipid content complicates the treatment even more. These lipids are typically in the form of metal salts additionally providing high metal content into oil. Traditionally, these phospholipids as such have been removed before further processing whereby usable lipid content is lost. The extraction of oil at a high temperature produces oil with less impurities. However, many valuable ingredients contained in microbial and algal biomasses are destroyed at these high temperatures. Therefore, in order to preserve the value of the residual biomass, the oil extraction should be carried out at mild temperature conditions. Unfortunately, the oil resulting from solvent extraction in mild process temperatures of, for example, 20° C.-150° C. usually results in a product rich in metals and phosphorus impurity content. These type of oils can also be very difficult to handle and purify by traditional means such as degumming because of the presence of emulsifying compounds, such as high level of phospholipids. Merely, the typical high original amount of phospholipid in algal oil results in decreasing the oil yield when using degumming resulting in ineconomical processing.
US2009/0266743 discloses a method for thermally treating triglyceride or triglyceride/hydrocarbon mixture for decreasing the metal and phosphorus content. In this method hydrocarbon which has a boiling point from about 25° C. to about 760° C. including a large variety of hydrocarbon compounds and mixtures and a triglyceride are passed through a heating zone. The temperature in this zone is from about 40° C. to about 540° C. A feed is produced which is contacted with a hydrotreating catalyst in a reaction zone to produce a reaction product containing diesel boiling range hydrocarbons.
WO2008034109 discloses a method for recovering fatty acids in form of alkyl esters from microbial biomass, such as microalgae, bacteria and fungi. The wet biomass is treated at high temperatures up to 450° C. and elevated pressure, such as up to 40 MPa (about 400 bar). This high temperature treatment aims at and results in disruption of the cells and formation of an oily phase. An alcohol, such as methanol or ethanol, is added to the oily phase and reacted therewith forming alkyl esters (FAME or FAEE). Co-solvents, such as alkanes, and catalyst, such as organic acids, can be used. Esterification reactions require essentially water free environment and high amount of alcohol present.
Degumming is the process of removal of phospholipids, including gums, typically from vegetable crude oil or edible oil wherein they are dissolved. Especially hydratable phospholipids may be removed by treatment with hot water. Oil containing non-hydratable phospholipids require use of an acid, such as phosphoric acid. Vegetable oils wherefrom hydratable phosphatides have been eliminated by a aqueous degumming process, may be freed from non-hydratable phosphatides by for example enzymatic treatment.
Total hydrolysis of lipids to obtain free fatty acids is well known and can be performed, for example, by treatment with water i.e. hydrotreatment. Acylglycerols and phospholipids have been successfully split or decomposed by hot pressurized water into free fatty acid. Water simultaneously splits phospholipids and glycerides to phosphate, glycerol and free fatty acids. However, free fatty acids are known to be corrosive and causing problems in subsequent processing. Therefore, extensive formation of free fatty acids should be avoided.
EP2097496 discloses a process for direct conversion of lipidic biomass to a transportation fuel. In this process lipidic biomass comprising glycerides or materials resulting in triglycerides is thermally hydrolysed with liquid water at about 220-300° C. Glycerides and other oily components are totally decomposed into free fatty acids and glycerol. The obtained free fatty acids are processed further into jet fuel, gasoline or diesel and glycerol is used as a combustable heat source in the treatment process.
Prior art provides means for treating oily biomass by conversion into esters or splitting into free fatty acid. However, it would be preferred to obtain as high glyceride content for the recovered oil as possible because of the corroding nature of free fatty acids.
On the other hand, problematic phospholipids and other complex lipids can be completely removed by degumming, which however significantly lowers the yield. In degumming the complete complex lipid is removed intact i.e. without decomposition or structural decay thus lowering the yield of lipidic material suitable for feedstock in further fuel production. For example, phospholipids typically contain two long chain fatty acids which are attached to the glycerol back bone and are suitable for feedstock in fuel production. There remains a need for recovery of the lipid components as intact as possible from the phospholipids to enhance the overall quality of the recovered usable oil fraction.